A high-frequency circuit of wireless terminals such as mobile phones has a duplexer including a transmission filter and a reception filter connected to a common antenna terminal. Acoustic wave filters using surface acoustic waves (SAW: Surface Acoustic Wave), or bulk acoustic waves (BAW: Bulk Acoustic Wave) are used to transmission and reception filters, and it is determined which kind of acoustic wave filter is used based on a specification of a device. For example, both of the transmission filter and the reception filter may be a SAW filter or a BAW filter, or one may be a SAW filter and the other may be a BAW filter.
The BAW filter is formed by the combination of piezoelectric thin film resonators having a lower electrode, a piezoelectric thin film, and an upper electrode stacked on a substrate in this order. There has been known a piezoelectric thin film resonator of an FBAR (Film Bulk Acoustic Resonator) type having a space formed below a region in which a lower electrode and an upper electrode face each other and a piezoelectric thin film resonator of an SMR (Solidly Mounted Resonator) type having an acoustic reflection film instead of the space. In addition, there has been known a temperature compensated type TC-FBAR (Temperature Compensated FBAR) including a temperature compensation film that has a temperature coefficient of a resonance frequency opposite in sign to that of the piezoelectric thin film and is inserted into an FBAR. The TC-FBAR has been known to have a tradeoff relation between a temperature coefficient of frequency TCF of a filter and an effective electromechanical coupling coefficient k2eff.
It is preferable that the filter of the duplexer has a steep skirt characteristic at a side of a guard band between transmission and reception filters and a good temperature stability in order to suppress the interference of signals between the transmission filter and the reception filter. However, when the TCF is improved in order to improve temperature stability, the k2eff decreases and the bandwidth of the filter becomes narrow due to the tradeoff relation between the TCF and the k2eff described above.